JP7844584B2 - Background of the method for producing purified precipitated calcium carbonate from limestone mud - Google Patents

Description

This disclosure generally relates to a method for processing lime mud cake generated as a waste product to produce precipitated calcium carbonate, and more particularly to a method for processing lime mud cake generated as a waste product to produce precipitated calcium carbonate suitable for use as a filler and/or pigment in paper and cardboard.

Brief Explanation of Related Technologies: The main component of paper and cardboard is cellulose pulp fiber, produced from wood or other plant sources through various mechanical and/or chemical pulping processes. The primary chemical pulping process used in the paper industry is the alkaline "kraft" process, which uses sodium hydroxide (caustic soda) and sodium sulfide in the digestion process to extract and separate non-cellulose materials from cellulosic pulp fibers. Another common variation in pulping, in particular, is the absence of sodium sulfide, which is known as caustic pulping.

To maximize the operational and economic efficiency of the pulping process, chemicals are recovered and reused whenever possible. This chemical recovery process includes the use of lime ovens in some pulping operations. Lime ovens function to produce lime (CaO) that is combined (deactivated) with "green liquor" in a process called causticization. Green liquor originates from the pulping process between pulping and causticization. If a lime oven is unavailable, commercially available lime (CaO) is purchased and used in a single pass of the causticization circuit.

The main component of the green liquor is sodium carbonate, and the reaction between lime and sodium carbonate produces sodium hydroxide (caustic soda) and calcium carbonate ( _CaCO3 ) through the following reaction steps.
CaO+ _H2O →Ca(OH) ₂ (1)
Ca(OH) ₂ +Na ₂ CO ₃ →CaCO ₃ +2NaOH (2)

The calcium carbonate produced in step 2 is called "lime mud" in the industry, and the sodium hydroxide (caustic soda) solution is called "white liquor." Due to the reaction conditions in step 2, the lime mud precipitates as relatively large particles that can be quickly separated from the white liquor flow. After washing and filtration, the lime mud is recycled to a lime kiln if present, or disposed of in a landfill. In practice, systems often contain both recycled lime mud and some landfilled purged lime mud.

Paper and cardboard products often contain calcium carbonate as fine particles produced from crushed limestone or synthetically precipitated as a filler, and/or as a coating pigment, due to its inherently superior whiteness and brightness compared to other mineral pigments. Despite containing calcium carbonate, lime mud is generally unsuitable for use as a filler and/or coating pigment in paper and cardboard. Lime mud has relatively low whiteness and brightness. Furthermore, lime mud generally has a large particle size and high pH due to residual caustic soda. Lime mud produced by pulping non-wood species such as reeds and straw often contains elevated levels of high-surface-area siliceous minerals, which increase the specific surface area of the lime mud particles, making them unsuitable for use as a filler in conventional papermaking.

Another drawback of lime mud is that the black particles of char can mix with the lime mud particles. Black char can be generated from combustion in the recovery boiler, which can be carried to the green liquor, and ultimately to the lime mud produced from the caustication of the green liquor. Black char, which can produce black spots, is highly undesirable for white pigments intended for use in paper. For these reasons, lime mud is disposed of in landfills unless it is re-combusted in a lime kiln to produce calcium oxide (CaO) for reuse in the caustication process. Even when recycling lime mud, it may be advantageous to purge more lime mud to (i) increase pulping capacity, or (ii) clean up the system of non-process elements to improve process efficiency.

It would be beneficial if lime mud could be processed and made suitable for use as a filler and/or coating pigment in paper and cardboard, rather than being sent to landfills. This would also improve the overall utilization rate of lime purchased by paper mills and reduce the consumption of purchased mineral fillers and/or coating pigments. Furthermore, refining this lime mud into viable new products would support global corporate efforts to minimize waste and promote a circular economy where resource use is more efficient through the reuse, recycling, or repurposing of materials.

According to embodiments of the present disclosure, a method for producing purified precipitated calcium carbonate from lime mud may include: mixing a lime mud cake with water and sodium carbonate to form a first slurry; heating the first slurry under conditions that allow the slurry to mature and form one or more of pyrsonite, shortite, and geyrasite; separating the solid portion from the matured slurry; washing the solid portion under conditions sufficient to decompose one or more of pyrsonite, shortite, and geyrasite into a _CaCO3 solid fraction and _{a Na2CO3} solid fraction and remove the sodium salt; and mixing the _CaCO3 solid fraction with water and a dispersant to disperse the _CaCO3 solid fraction in water to form a dispersed slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm, thereby producing a dispersed slurry containing purified precipitated calcium carbonate.

According to embodiments of this disclosure, a method for producing purified precipitated calcium carbonate from lime mud may include: mixing a lime mud cake with water and a dispersant to form a first slurry having a Brookfield viscosity of less than 1000 cps at 100 rpm; grinding the first slurry to a median particle size of about 0.4 microns to about 5 microns; phase-separating the ground slurry under conditions sufficient to obtain a centrifugal slurry containing impurity particles and a paste containing purified calcium carbonate; and diluting the paste in water to a target solids content, thereby producing a dispersed slurry containing purified precipitated calcium carbonate.

According to embodiments of this disclosure, a method for producing purified precipitated calcium carbonate from lime mud may include: mixing a lime mud cake with water to form a first slurry; adjusting the pH of the first slurry to about 10 to about 11; centrifuging the first slurry over a residence time of about 1 to about 10 minutes under conditions sufficient to achieve a g-force of about 500 to about 2000 g to obtain a centrifugal slurry containing impurity particles and a paste containing purified calcium carbonate; and mixing the paste with water and a dispersant to form a dispersed slurry having a Brookfield viscosity of less than about 1000 cps at 100 rpm and containing purified precipitated calcium carbonate.

According to embodiments of this disclosure, a method for producing purified precipitated calcium carbonate from lime sludge comprises: mixing a lime sludge cake with water to form a first slurry; adjusting the pH of the first slurry to approximately 8 to approximately 11; mixing the first slurry with a silicate flotation agent compound to form a second slurry; processing the second slurry in a flotation cell system under conditions sufficient to form a concentrated foam containing silicate particulate impurities and a tail slurry containing CaCO3; washing and separating the tail slurry to obtain a liquid phase containing excess soluble salts and a paste containing purified CaCO3; and mixing the paste with water to form a dispersed slurry having a viscosity of less than approximately 100 cps at 100 rpm and containing purified precipitated calcium carbonate.

According to embodiments of the present disclosure, a method for removing blackchar from a starting slurry containing calcium carbonate and blackchar comprises: flowing the starting slurry through a first liquid cyclone under conditions sufficient to cause the blackchar particles to rise to the top to form a first overflow and to retain the calcium carbonate at the bottom to form a first underflow; flowing the first underflow through a second liquid cyclone under conditions sufficient to cause the blackchar particles to rise to the top to form a second overflow and to retain the calcium carbonate at the bottom to form a second underflow; and causing the blackchar particles to rise to the top to form a third overflow and to remove the calcium carbonate This may include: directing the first overflow into a third liquid cyclone under conditions sufficient to retain it at the bottom and form a third underflow; directing the second underflow into a recovery chamber; directing the second overflow into the first liquid cyclone; directing the third underflow into the first liquid cyclone under conditions sufficient to cause the blackchar particles to rise to the top and form a fourth overflow, and to retain the calcium carbonate at the bottom and form a fourth underflow; directing the fourth overflow into a waste container; and directing the fourth underflow into the third liquid cyclone.

According to embodiments of this disclosure, a method for removing blackchar from a starting slurry containing calcium carbonate and blackchar may include: flowing the starting slurry through a liquid cyclone under conditions sufficient to cause the blackchar particles to rise to the top to form an overflow and to retain the calcium carbonate at the bottom to form an underflow; flowing the overflow to a waste disposal system; and flowing the underflow to a recovery chamber.

According to embodiments of this disclosure, a method for removing blackchar from a starting slurry containing calcium carbonate and blackchar may include: flowing the starting slurry through a first liquid cyclone under conditions sufficient to cause the blackchar particles to rise to the top, forming a first overflow, and to retain the calcium carbonate at the bottom, forming a first underflow; flowing the first underflow through a second liquid cyclone under conditions sufficient to cause the blackchar particles to rise to the top, forming a second overflow, and to retain the calcium carbonate at the bottom, forming a second underflow; discharging the first and second overflows to waste; and discharging the second underflow into a recovery chamber.

According to embodiments of this disclosure, a method for removing blackchar from a starting slurry containing calcium carbonate and blackchar may include: loading the starting slurry into a trap tank using a continuous inflow at or near the center of the trap tank; agitating the starting slurry in the trap tank using a stirrer for a residence time of about 4 to 10 minutes at a tip velocity of about 0.1 m/s to about 1.5 m/s, under conditions sufficient to cause the blackchar particles to rise to the top of the trap tank and the calcium carbonate to settle at the bottom of the trap tank; and discharging the calcium carbonate from the bottom of the trap tank into a recovery container.

According to embodiments of this disclosure, a method for removing blackchar from a starting slurry containing calcium carbonate and blackchar includes flowing an ozone-containing gas through the starting slurry at a flow rate of approximately 0.1 liters per minute to approximately 2 liters per minute of the starting slurry using a stirrer at a tip speed of approximately 1 m/s to approximately 5 m/s, thereby oxidizing the blackchar to carbon dioxide gas by the ozone and removing it with the gas flow.

According to embodiments of this disclosure, a method for removing blackchar from a starting slurry containing calcium carbonate and blackchar may include: mixing the starting slurry with a foaming agent and a flotation agent compound to form a second slurry; processing the second slurry with a flotation device, under an airflow of approximately 1 slpm to approximately 3 slpm per liter of the second slurry, using a stirrer at a tip speed of approximately 150 m/min to approximately 500 m/min for approximately 1 to 10 minutes, together with the foam overflowing from the flotation device, the tail slurry remaining in the flotation device, the foam containing blackchar, and the tail slurry containing calcium carbonate; and collecting the tail slurry and dispersing the tail slurry in water to form a dispersed slurry containing calcium carbonate.

In any of the aforementioned embodiments of the method for removing blackchar, the starting slurry may be a slurry of the purified precipitated carbonate product. For example, the starting slurry may be a slurry of the purified precipitated carbonate product resulting from any of the methods disclosed herein.

This is a schematic diagram of a trap tank according to an embodiment of the present disclosure. This is a schematic diagram of a trap tank according to an embodiment of the present disclosure. This is a simplified flow scheme showing four liquid cyclones for removing black char from waste. Field emission scanning electron microscope images of a limestone mud sample are shown at two magnifications. The higher magnification shows how impurities (plate-like and very fine particles) are embedded within the calcium carbonate aggregates.

Methods for processing lime sludge according to embodiments of the present disclosure may include a series of chemical and mechanical treatments. The methods according to the present disclosure process lime sludge waste products into particles of a suitable diameter and purity for use as fillers and/or pigments in paper and cardboard products, and remove undesirable impurities such as mineral silicates and black carbonaceous materials. Precipitated calcium carbonate generally suitable for use as a filler or pigment may have a median particle size distribution of about 0.5 μm to about 5 μm, a specific surface area of about 3 ^m² /g to about 20 ^m² /g, and one or more ISO lightness values greater than about 80, and may be free of visible blackchar. Unless otherwise specified, the median particle size distribution of the obtained precipitated calcium carbonate is measured using a laser light scattering device such as a Horiba LA-950. Sample preparation for particle size measurement includes mixing with a dispersant polymer such as poly(acrylic acid), followed by sonication.

A method for producing precipitated calcium carbonate from limestone mud according to this disclosure may include either or both of the removal of impurities having a high specific surface area and/or the removal of visible blackchar. A method for producing precipitated calcium carbonate by the removal or reduction of impurities having a high specific surface area and/or the removal of visible blackchar may include a two-step process. In embodiments, the method of this disclosure may produce purified precipitated calcium carbonate by removing high specific surface area material. In embodiments, the method may include further processing of the obtained purified precipitated calcium carbonate to remove visible blackchar for further purification of the purified precipitated calcium carbonate. In embodiments, the method of this disclosure may produce purified precipitated calcium carbonate by removing blackchar.

High specific surface area impurities may include one or more of silicates, calcium silicate hydrates, hydrotalcite-like compounds, calcium aluminate, calcium phosphate, and amorphous silicates. Generally, high specific surface area refers to impurity particles having a specific surface area greater than 20 ^m² /g. For example, high specific surface area materials may have a surface area of more than 20 ^m² /g to 100 ^m² /g or more. For instance, a typical surface area of such a high specific surface area material is approximately 40 ^m² /g to approximately 60 ^m² /g.

Blackchar is a black carbonaceous substance that can be present in limestone mud. Depending on the source of the limestone mud, it may be necessary to treat the mud to remove both high-surface-area impurities and blackchar. Alternatively, only blackchar removal may be required. When used in combination with methods for removing high-surface-area materials, blackchar removal can be performed after the removal of the high-surface-area materials. Generally, blackchar removal is carried out until no visible blackchar remains.

Embodiments of this disclosure include the removal of high surface area impurities by one or more flotation cell methods, thermal aging methods, and phase separation methods. Embodiments of this disclosure may separately or additionally include the removal of visible blackchar by one or more of the liquid cyclone method, trap tank method, ozone method, and flotation method. Any one or more suitable combinations of any one or more high surface area impurity removal methods and visible blackchar removal methods may be utilized.

In any of the methods described herein, lime mud cake may be produced as a waste product in many processes, such as paper kraft pulp mills and sugar beet production, and as a by-product of acetylene production. Lime mud may be a reaction product of reacting lime and green liquor from recovered boiler smelting. The smelting may be obtained from the combustion of kraft process black liquor or soda process (NSSC) black liquor, or from chemical mechanical pulp liquid (CMP, CTMP, APMP).

In any of the methods disclosed herein, the lime mud cake may be cleaned by mixing the lime mud cake with water to form a first slurry, and then washing the first slurry to remove the caustic soda present in the lime mud cake.

In any of the methods of pH adjustment described herein, the pH may be lowered by passing carbon dioxide gas and/or a carbon dioxide-containing gas through the slurry. For example, flue gas may be used. Other suitable gases include pure carbon dioxide gas from liquefaction sources, lean combustion exhaust gas, ethanol, or carbon dioxide by-products from petrochemical sources. The flue gas may be from a boiler or furnace with a carbon dioxide content of approximately 8% to 30%.

In any of the methods of grinding described herein, grinding can be achieved by any known suitable method for fine grinding. For example, the slurry can be ground by various known grinding techniques such as ball mills, sand mills, and media mills. For example, the slurry can be ground using a vertically or horizontally agitated media mill using glass, sand, and/or ceramic media. The media may have a median diameter of about 0.5 mm to about 3 mm. The slurry is ground to a particle size suitable for the expected end use. For example, when processing lime mud to form precipitated calcium carbonate, the method of the disclosure may include grinding to median particle sizes of about 0.5 microns to about 5 microns, about 1 micron to about 2 microns, about 0.5 microns to about 1 micron, about 3 microns to about 5 microns, or about 2 microns to about 3 microns. Suitable diameters include median particle sizes of about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 microns.

Flotation Cell Method In embodiments, a method for producing precipitated calcium carbonate from lime sludge may include the use of a flotation cell to remove high specific surface area impurities from the lime sludge. Flotation cell technology is commonly used to remove larger particle size impurities with an average particle diameter of about 50 to 500 microns. It has been advantageously found that the method according to this disclosure may enable the use of flotation cell technology to remove smaller particle size impurities, such as those with a particle diameter of 4 to 20 microns. Referring to Figure 3, high surface area materials generally bind to calcium carbonate crystals as aggregates. Surprisingly, despite these aggregates, it has been found that the separation of such impurities can be achieved by the flotation cell method of this disclosure. Those skilled in the art would not have expected the ability to separate such aggregates without removing or adversely affecting calcium carbonate using conventional flotation cell technology or even centrifugation.

This method may involve mixing the lime clay cake with water to form a first slurry and adjusting the pH to about 8 to about 11, or about 9 to about 10.5, or about 8 to about 10, or about 8 to about 11. Other suitable pH values include about 8, 8.5, 9, 9.5, 10, 10.5, and 11. Next, the first slurry may be mixed with a silicate flotation agent compound to form a second slurry, and the second slurry may be diluted with water to achieve a solids content of about 5% to about 15% by weight based on the total weight of the second slurry. Alternatively, a solids content of about 5% to about 15% by weight based on the total weight of the slurry can be achieved by dilution when mixing the lime clay cake with water to form the first slurry.

Next, the second slurry can be processed through a flotation cell system under conditions sufficient to form a concentrated foam containing silicate particle impurities and a tail slurry containing _CaCO3 .

The tail slurry can be washed and separated into a liquid phase containing excess soluble salts and a paste containing purified _{CaCO3 .} The paste can be mixed with water and a dispersant to form a dispersion slurry having a viscosity of less than about 1000 cps at 100 rpm.

The purified precipitated calcium carbonate product yields a dispersed slurry. If required for a specific application, the dispersed slurry can be ground to a median particle size of approximately 0.5 to 5 microns, and/or, if the pH is outside this range after grinding, the pH can be adjusted to approximately 9 to 10.5.

In embodiments, the first slurry containing the silicate flotation agent compound may be mixed to a solids content of about 5% to about 40% by weight, based on the total weight of the slurry. Other suitable ranges include about 5% to about 20% by weight, about 5% to about 10% by weight, about 20% to about 25% by weight, and about 30% to about 40% by weight. Other suitable values, based on the total weight of the first slurry, include about 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 37, 38, and 40% by weight.

Silicate flotation agents can be amine-based compounds. For example, a silicate flotation agent may be one or more of primary amines, dialkylamines, tertiary amines, and quaternary amines. Examples of specific compounds include, but are not limited to, animal fats, cocoamines, hydroxyethylalkylimidazolines, laurylamines, long-chain alkylpyridiniums, and n-alkyltrimethylammonium. Silicate flotation compounds are commercially available and may include one or more of the following: Flotigam® 3135 (Clariant), Flotigam® K2C (Clariant), Armak® 1019 (Akzo Nobel Surface Chemistry, LLC), MD 20763 (Akzo Nobel), CustAmine® 1208 (ArrMaz), Tomamine® DA-17 (Evonik Industries), Ethhomeen® (Akzo Nobel), and Duomac-T® (Akzo Nobel).

The silicate flotation agent compound may be provided in the first slurry in an amount of about 0.1% to about 0.3% by weight, based on the dry mass of the lime mud cake.

Thermal Aging Method According to one embodiment, a method for producing precipitated calcium carbonate from lime mud may include thermal aging of the lime mud in sodium carbonate to reduce high surface area impurities. In the embodiment, the method may include mixing a lime mud cake with water and sodium carbonate to form a first slurry, and heating the first slurry under conditions that allow the slurry to mature and form one or more of the following: pyrsonite ( _Na₂Ca (CO₃) _₂ ), _2H₂O , _schonotite ( _Na₂Ca₂ ( _CO₃ ) _₃ ), and gaylasite ( _Na₂Ca ( _CO₃ ) _{₂.5H₂O )} . The method may further include separating the solid portion from the matured slurry, and washing the solid portion under conditions that decompose one or more of the pyrsonite, schonotite, and gaylasite into a _CaCO₃ solid fraction and a _{Na₂CO₃ solution} fraction, and remove sodium salts. Next, the _CaCO3 solid fraction can be mixed with water and a dispersant to disperse the _CaCO3 solid fraction in water, forming a dispersed slurry having a Brookfield viscosity of less than approximately 100 cps at 100 rpm. Unless otherwise specified, the viscosity values reported herein are Brookfield viscosity values. _{The CaCO3} solid fraction is, for example, purified precipitated calcium carbonate with a specific surface area suitable for use in papermaking applications.

If required for the desired application, the dispersion slurry may then be ground to a median particle size of approximately 0.5 to 5 microns, and/or the pH may be adjusted to approximately 8 to 11. For example, the pH may be approximately 9 to 10, or approximately 9 to 10.5, or approximately 8 to 10, or approximately 8 to 11. Other suitable pH values include approximately 8, 8.5, 9, 9.5, 10, 10.5, and 11.

In embodiments, the process may include washing the first slurry before thermal aging.
The first slurry may be cleaned, for example, using a filter press, a purification device, and/or a rotary vacuum filter. In embodiments, cleaning may be achieved using water. For example, an amount of water equal to about 1 to about 5 times the dry mass of solids in the first slurry may be used.

In some embodiments, thermal aging can be performed by heating the slurry to a temperature of approximately 80°C to 130°C, approximately 90°C to 100°C, approximately 85°C to 95°C, or approximately 80°C to 90°C. Other suitable temperatures include approximately 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, and 130°C.

In this embodiment, the slurry may be aged for approximately 2 to 8 hours, 2 to 4 hours, 3 to 7 hours, 5 to 8 hours, or 3 to 6 hours. Other suitable times include approximately 2, 3, 4, 5, 6, 7, or 8 hours.

In embodiments, the first slurry may contain about 20% to about 40% by weight of sodium carbonate, based on the total weight of the first slurry. Other suitable amounts of sodium carbonate include about 25% to about 40% by weight, about 30% to about 35% by weight, about 20% to about 30% by weight, or about 25% to about 35% by weight. For example, the first slurry may contain about 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, or 40% by weight, based on the total weight of the first slurry.

In embodiments, the first slurry may contain about 5% to about 30% by weight of lime mud, based on the total weight of the first slurry. Other suitable amounts of lime mud include about 5% to about 15% by weight, about 10% to about 20% by weight, and about 15% to about 30% by weight. For example, the first slurry may contain about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30% by weight.

In this embodiment, the solid portion is separated from the matured slurry and then washed with water.
Washing can be performed by passing water through the cake or paste using known equipment. For example, a filter press may be used. Alternatively, the solid portion may be resuspended in water and the separation process may be repeated. Other known washing techniques and equipment may be used.

In embodiments, the sodium carbonate used in the process may be recycled for subsequent use. However, before recycling in the process, dissolved silica present in the sodium carbonate must be removed from the process. This can be done, for example, by lowering the pH of the recovered sodium carbonate to about 9.5 to precipitate the silica, and then filtering off the precipitated silica using any suitable filtration method. In embodiments, dissolved silica can be removed by mixing the recovered sodium carbonate with a sodium aluminate solution to precipitate the aluminosilicate, and then filtering off the aluminosilicate using any suitable filtration method known in the art.

Phase Separation Method: According to one embodiment, a method for producing purified precipitated calcium carbonate from limestone mud may involve the use of phase separation to remove high-surface-area impurities. It has been advantageously found that high-surface-area impurities can be phase-separated and removed from the calcium carbonate to produce a purified precipitated calcium carbonate product.

In embodiments, this method may include mixing a lime clay cake with water and a dispersant to form a dispersion slurry having a viscosity of less than about 1000 cps at 100 rpm, and adjusting the pH of the dispersion slurry to about 8 to about 10.5. In embodiments, the dispersion slurry may have a viscosity of less than about 100 cps at 100 rpm. This method may further include grinding the dispersion slurry to a median particle size of about 0.5 microns to about 5 microns. After grinding, the pH may be readjusted to about 8 to about 10.5 as needed. Next, this method may include inducing phase separation in the grinding slurry using conditions sufficient to obtain a slurry containing impurity particles and a paste containing purified calcium carbonate. Precipitated calcium carbonate can be produced by separating the paste from the centrifugated product and dispersing it in water to the desired solid content.

In embodiments, the method may include mixing a lime clay cake with water to form a first slurry, the pH of which may be adjusted to about 10 to about 11, inducing phase separation to separate impurities and produce a paste containing purified calcium carbonate. The paste may then be separated and mixed with water and a dispersant to form a dispersion slurry having a viscosity of less than about 1000 cps at 100 rpm. In embodiments, the dispersion slurry may have a viscosity of less than about 100 cps at 100 rpm. The dispersion slurry can be ground to a particle size of about 0.5 microns to about 5 microns, and the pH can be adjusted to about 9 to about 10.5. In such embodiments where grinding is performed after the phase separation step, it has been found that the dispersant is not required in the first slurry and can instead be added when forming the dispersion slurry of purified precipitated calcium carbonate.

In any of the phase separation methods disclosed herein, centrifugation may be used as the phase separation method. For example, this method may involve centrifuging a pulverized slurry over a residence time of about 1 to about 10 minutes under conditions sufficient to achieve a g-force of about 500 to about 2000 g to obtain a centrifugal slurry containing impurity particles and a paste containing purified calcium carbonate. The paste may be separated from the centrifuged product and dispersed in water to a target solids content, depending on the final use of the purified precipitated carbonate product.

In any of the phase separation methods disclosed herein, impurities can be separated from calcium carbonate using gravity separation or other known phase separation techniques. Gravity sedimentation parameters, such as sedimentation time and vertical liquid depth, can be adjusted for a given slurry.

The first slurry may have a solid content of about 10% to about 35% by weight, based on the total weight of the slurry. Other suitable amounts include about 15% to about 30% by weight, about 10% to about 20% by weight, about 20% to about 25% by weight, or about 10% to about 30% by weight. For example, the first slurry may have a solid content of about 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, or 35% by weight.

In any of the embodiments described above, when the paste or slurry is diluted to form a dispersed slurry of purified precipitated calcium carbonate, the dispersed slurry may have a solid content of about 25% to about 50% by weight, based on the total weight of the dispersed slurry. Other suitable solid content by weight based on the total weight of the dispersed slurry include about 25% to about 40%, about 30% to about 45%, about 30% to about 50%, or about 40% to about 50%. For example, the dispersed slurry may have a solid content of about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, and 50% by weight, based on the total weight of the dispersed slurry.

In any of the embodiments described above, the dispersant may be one or more polycarbomote or copolymers of monomer units comprising sodium poly(acrylate), acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, hydroxyacrylic acid, and maleic anhydride.

Liquid cyclone treatment for removing blackchar. Liquid cyclone treatment can be used alone or in combination with any method for removing blackchar from lime mud by removing high surface area material. When used in combination with a method for removing high surface area material, blackchar can be removed after the removal of the high surface area material.

As described above, the blackchar removal method may be carried out on precipitated calcium carbonate or lime mud cake resulting from a treatment for removing high-surface-area impurities. When used with purified precipitated calcium carbonate after the removal of high-surface-area impurities, the resulting product is generally a slurry of purified precipitated calcium carbonate, water, and a dispersant. When used with lime mud cake, the lime mud cake may be dispersed in water with an optional dispersant to form a slurry for treatment by the blackchar removal process. Any of the above-mentioned dispersants may be used. For ease of reference, the term “blackchar removal starting slurry” is used herein and is understood to refer to a slurry formed from a pre-treated precipitated calcium carbonate slurry or lime mud cake from which high-surface-area impurities have been removed.

A blackchar removal starting slurry containing water, a dispersant, and blackchar impurities may be passed through an apparatus comprising one to four liquid cyclones in a series and/or parallel configuration. As the slurry passes through the apparatus, the blackchar impurities float to the top of the slurry and can be removed. Referring to Figure 2, for example, in an embodiment with four liquid cyclones, the process may include passing the slurry through a first liquid cyclone, where the overflow (lighter particles) moves to a third liquid cyclone and the underflow (heavier particles) moves to a second liquid cyclone. In the second liquid cyclone, the overflow moves to the first liquid cyclone, while the underflow moves to a recovery chamber. In the third liquid cyclone, the overflow moves to a fourth liquid cyclone, while the underflow moves to the first liquid cyclone. In the fourth liquid cyclone, the overflow moves to waste, and the underflow moves to the third liquid cyclone.

Trap Tank Treatment for Blackchar Removal Instead of liquid cyclone treatment, blackchar can be removed using trap tank treatment. Schematic images of a trap tank are shown with reference to Figures 1A and 1B. Trap tanks may generally have a conical shape. In the embodiments shown in Figures 1A and 1B, the trap tank has an upper cylindrical portion located at the base of a frustoconical shape. The trap tank may further include an overflow valve, rotating blades, and an outlet. The overflow valve may be located in the upper region of the trap tank where the slurry is supplied to the trap tank, and may collect overflow that may result from having a supply rate higher than the output rate, for example. While not intended to be constrained by theory, trap tank design is thought to balance the slow downward flow of slurry with the upward tendency towards the surface in the blackchar slurry. Agitation near the surface of the slurry promotes the separation of blackchar from the lime mud particles.

In embodiments such as those shown in Figures 1A and 1B, the cylindrical portion may include a supply line for supplying a blackchar removal starting slurry from a supply tank or other receptacle, and a rotating blade located at the bottom of the cylindrical portion for agitating the slurry once it is supplied to the trap tank. As the slurry in the trap tank is agitated, blackchar impurities rise to the surface, leaving the purified sample at the bottom of the tank. An outlet pump may be incorporated at the bottom of the trap tank to recover the purified sample. In embodiments, the system may be operated to maintain a constant or substantially constant flow of feed to and from the trap tank. In embodiments, this method may include passing the recovered sample through the trap tank one or more, two or more, three or more, four or more, or five or more times. For example, about one to three times, about two to five times, about one to four times, and about one to five times. Once the supplied/recovered sample has passed through the trap tank the desired number of times, it may be collected in a recovery chamber. If necessary, the recovered product can be further processed to adjust the pH and/or the solids content, making it more suitable for use as a filler or pigment.

The agitator may be a flat disc-shaped blade, a dispersion blade, and/or a saw-toothed impeller. The agitator may be any blade capable of achieving laminar flow in the slurry. In embodiments, the agitator may be a cowl blade having a diameter equal to about 0.4 to about 0.95 times the diameter of the cylindrical section of the trap tank.

The agitator can rotate at a tip speed of approximately 0.1 to 1.5 m/s while maintaining laminar flow.

In this embodiment, the slurry may have a residence time in the tank of approximately 2 to 10 minutes, 5 to 8 minutes, 4 to 6 minutes, 3 to 7 minutes, or 2 to 9 minutes. Other suitable residence times include approximately 2, 3, 4, 5, 6, 7, 8, 9, or 10 minutes.

Ozone Method for Blackchar Removal In this embodiment, blackchar can be removed by exposing a blackchar removal starting slurry to ozone treatment. The blackchar removal starting slurry may be or contain purified precipitated calcium carbonate resulting from any of the methods described above. Exposure to ozone may be carried out while stirring the blackchar removal starting slurry. Although not intended to be bound by theory, it is thought that ozone oxidizes the blackchar to carbon dioxide, which is then removed in a gaseous stream.

Any suitable apparatus can be used to generate and flow ozone through the blackchar removal starting slurry. The generating apparatus may use ultraviolet light, electrical discharge, or electrolysis. Ozone can be generated in flowing air or pure oxygen, or a mixture of the two. The air or oxygen may be flowed at a flow rate of about 0.1 to about 2 liters/minute of ozone-containing gas per liter of lime sludge slurry. For example, 675 mL of lime sludge slurry with a solid content of 10% can be treated using a dry air flow of about 1 liter per minute. The concentration of ozone in the air or oxygen flow may be in the range of about 2 g/ ^m³ to 50 g/ ^m³ , about 25 g/ ^m³ to about 50 g/ ^m³ , about 5 g/ ^m³ to about 12 g/ ^m³ , ^about 2 g/ ^m³ to about 15 g/ ^m³ , about 8 g/ ^m³ to about 15 g/m³, or about 35 g/ ^m³ to about 45 g/ ^m³ . Other suitable concentrations include approximately 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, ⁴⁸ , and 50 g/m³.

This process can be carried out using any agitator design suitable for maximizing gas contact with the liquid. For example, radial flow or Rushton impellers can be used at tip speeds of approximately 1 m/s to approximately 5 m/s. The ozone reaction can be carried out at high temperatures and high pH. For example, a temperature of 40°C can be used. For example, temperatures of approximately 40°C to 80°C, 50°C to 60°C, 40°C to 60°C, 50°C to 70°C, or 45°C to 65°C can be used. For example, pH above 10 or above 11 can be used. Higher reaction rates with ozone are achieved under these higher temperature and pH conditions, thus allowing for shorter reaction times in the container.

Flotation method for removing blackchar In this embodiment, blackchar can be removed using flotation. The blackchar removal starting slurry can be mixed with a foaming agent and a flotation collector compound. The mixture can be mixed under an airflow for 1 to 10 minutes at a tip velocity of about 150 m/min to about 500 m/min, about 200 m/min to about 350 m/min, about 300 m/min to about 400 m/min, or about 150 m/min to about 250 m/min, while collecting the foam as it overflows. The blackchar is removed by the foam. The remaining tail slurry contained purified precipitated calcium carbonate.

Suitable blowing agents include standard alcohols having hydrocarbon chains with 5 to 10 carbon atoms, such as methyl isobutylcarbinol (MIBC), amyl alcohol, cresol, and terpineol. Other types of blowing agents, such as polyalkoxy ethers and polyglycol ethers, may also be used. Any combination of one or more blowing agents may be used. The blowing agent may be used at levels of 10 to 250 ppm per dry weight of lime mud, or about 10 ppm to about 100 ppm, or about 100 ppm to about 250 ppm. Other preferred values include blowing agents at about 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 125, 150, 175, 200, 225, and 250 pp per dry weight of lime mud.

Suitable flotation agent compounds include kerosene or other oily compounds that are immiscible with water, such as diesel fuel. The agent may be used at concentrations of 100–1000 ppm, 200–500 ppm, about 150–350 ppm, or about 700 ppm–1000 ppm per dry mass of limestone mud. Other appropriate values include collecting agents at approximately 100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 625, 650, 675, 700, 725, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, and 1000 ppm.

The airflow can be approximately 1 standard liter/min (SLPM) to approximately 6 SLPM, approximately 3 SLPM to approximately 5 SLPM, approximately 1 SPLM to approximately 2 SLPM, or approximately 4 SLPM to approximately 6 SLPM. Other suitable flow rates include approximately 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8, 5, 5.2, 5.4, 5.8, and 6 SLPM.

In any embodiment of this specification, whether or not the precipitated calcium carbonate product and/or slurry are further treated for the removal of blackchar, they may be mixed with any additives for incorporating the precipitated calcium carbonate into a suitable formulation for use as a filler and/or pigment for a given papermaking application.

Example 1: Thermal aging (HA) of sodium carbonate
Lime mud was treated to obtain a precipitated calcium carbonate material having a reduced specific surface area and reduced impurity content compared to the starting material. The lime mud cake was obtained as waste product from a pulp mill. The lime mud cake was slurryed in a 25-30% sodium carbonate solution with a calcium carbonate solid content of 10-20%. In particular, 1 kg of slurry contained 200 g of _CaCO₃ (20% _CaCO₃ ), 240 g of _{Na₂CO₃ , and} 560 g of _H₂O (30% _{Na₂CO₃ )} .

Next, the slurry was heated and aged at 100°C for 8 hours to form the mixed salt pyrsonite ( _Na₂Ca ( _CO₃ ) _₂ · _5H₂O ). A comparison was conducted with a process without the heat aging step, and the results are summarized in the table below.

Next, the slurry was filtered or separated into solid and liquid phases by vacuum filtration using a Buchner funnel and paper. In the Buchner funnel, the cake was washed with three parts of water, each part having a solid mass equal to that of the original lime mud cake. The solids were washed with the removed water to remove the sodium salts, which can optionally be recycled at the start of the process. This washing also decomposed the pyrsonite back _{into CaCO₃} and _Na₂CO₃ .

Sodium poryl acrylic acid was used as a chemical dispersant to separate the _CaCO3 solids, which were then dispersed in water with a solid content of 35%. The viscosity of the slurry was reduced using the dispersant to a Brookfield viscosity of less than approximately 100 cps at 100 rpm.

The pH of the pulverized slurry was adjusted to approximately 9.

To recycle a sodium carbonate solution, the dissolved silica must first be removed, and the wash water separated. Silica can be removed by either: 1) lowering the pH to approximately 9.5 with carbon dioxide gas to form precipitated silica, which is then filtered off; or 2) precipitation as aluminosilicate using a sodium aluminate solution, which is also filtered off. Water can be removed by evaporation or membrane processes such as reverse osmosis.

The precipitated calcium carbonate formed by thermal aging showed a significant decrease in specific surface area and the amount of silicon dioxide present compared to treatment without the aging process.

Example 2: Removal of impurities using phase separation A lime clay cake obtained as a waste product from a pulp mill was slurryed in water with a solid content of 20%. The slurry was processed using a filter press equipped with a washing cycle to remove excess white liquor from the pulp mill. Using the filter press, a cake was first formed from the 20% solid slurry, and then water was passed through the cake. The washing cycle was performed using four times the mass of dry solids in water.

The washed cake was dispersed in water with a solid content of 20% using sodium polyacrylate as a chemical dispersant. The resulting dispersion slurry had a viscosity of approximately 10 cps at 100 rpm.

Next, the dispersed slurry was treated with combustion exhaust gas containing approximately 15% carbon dioxide to lower its pH to approximately 10.5.

Next, the pH-adjusted slurry was ground to a desired particle size suitable for use as a paper filler or coating pigment. In this example, the pH-adjusted slurry was ground using a vertical media mill operating in a single continuous pass. The resulting ground median particle size was approximately 3.5 microns.

The pH of the pulverized slurry was adjusted again by gas treatment with carbon dioxide to a pH of 9.5. The slurry was then continuously centrifuged at 1400–1500 rpm (see table), yielding a g-force of 713 or 819 g with a residence time of 4.8–8.4 minutes. The centrifuged material containing particulate impurities was removed, and the paste containing the purified product was separated for further processing.

Four samples were prepared using the method described above, and the pastes obtained from each sample had the solid content shown in the table below. The resulting precipitated calcium carbonate showed a significant reduction in impurity elements such as silicon, aluminum, magnesium, and iron. The reduction in specific surface area and _SiO₂ content compared to the starting kiln lime sludge is shown below.

Example 3: Removal of impurities using phase separation of lime sludge before grinding. Lime sludge cake obtained as waste product from a pulp mill was slurryed in water with a solid content of 20%. The slurry was processed using a horizontal filter press equipped with a washing cycle to remove excess white liquor from the liquid phase. Using the filter press, a cake was first formed from the 20% solid slurry, thereby obtaining a 65% solid cake, and then water was passed through the cake. The washing cycle was carried out using four times the mass of dry solids in water.

The obtained washed cake was slurryed in water with a solid content of 23%, and then treated with carbon dioxide containing combustion exhaust gas to lower the pH to 10.9.

Next, the pH-adjusted slurry was continuously centrifuged at 1300 rpm, and residence times were varied as shown in the table to obtain a g force of 615. The centrifuged material containing particulate impurities was removed, and the paste containing the purified product was separated for further processing.

The resulting centrifuged paste was dispersed in water with sodium poly(acrylate) as a dispersant, resulting in a solid content of 30%. The Brookfield viscosity of the dispersed slurry was 13 cps at 100 rpm. Four samples were prepared using the method described above, as shown in the table below.

The resulting product contained fewer impurity elements such as silicon, aluminum, magnesium, and iron. It also had reduced impurity phases such as calcium silicate, hydrotalcite, and blackchar, as well as a lower specific surface area.

Example 4: Removal of impurities using gravity sedimentation The process of Example 2 was repeated, except that a gravity sedimentation process was used instead of centrifugation after grinding. In particular, the centrifugation step in Example 2 was replaced by allowing the liquid to settle in a cylindrical test tube container at the indicated height and sedimentation time specified below. After the specified sedimentation time, the upper layer was drained and removed, and the lower layer was tested.

Gravity sedimentation resulted in a slight decrease in specific surface area. This was thought to be due to a reduction in impurities such as silicates.

Example 4: Flotation to remove impurities from lime sludge. Lime sludge cake, obtained as waste product from a pulp mill, was slurryed in water with a solid content of 20%. The slurry was then gas-treated with pure carbon dioxide to lower the pH to the pH values shown in the table.

For silicate-specific flotation collection, Flotigam® 3135 (CLARIANT) or CustAmine® 1208 (ARRMAZ) was added to the pH-adjusted slurry. The table below shows the dosage based on the dry mass of the lime sludge.

Next, the slurry was diluted with water to a solid content of 8% by weight and processed using a flotation cell system to obtain a tail slurry containing concentrated foam containing silicate particulate impurities and a purified product.

Next, the tail slurry was filtered, and excess soluble salts in the liquid phase were removed using a vacuum filter and paper. As a result of the treatment, a paste containing purified _CaCO3 was obtained.

The resulting product contained elements with few impurities, such as silicon, aluminum, magnesium, and iron. Furthermore, the impurity phases, such as calcium silicate and hydrotalcite, were reduced, resulting in a lower specific surface area. The table below shows the reduction of _SiO₂ achieved by this method.

Example 5: Removal of Blackchar by Liquid Cyclone Treatment The final product slurry, consisting of 30% calcium carbonate in water, sodium polyl (acrylic acid) as a dispersant, and blackchar impurities, was further treated to remove the blackchar impurities. The viscosity of the slurry was 40 cps. The slurry was treated with an apparatus containing four liquid cyclones (HCs). As the slurry passed through, the blackchar impurities floated to the top of the slurry and could be treated or separated from the purified recovered product (see Figure 2). The overflow (light particles) and underflow (heavy particles) from each HC flowed to the next HC as follows:

Qualitative observations showed that HC2 underflow resulted in less blackchar than feed products.

Example 6: Removal of Blackchar using a Trap Tank Blackchar was separated from the final product slurry containing 34.8% calcium carbonate in water, sodium boyl (acrylic acid) as a dispersant, and blackchar impurities using a trap tank as shown in Figures 1A and 1B. The slurry had a Brookfield viscosity of 20 cps at 100 rpm.

The supply line within the trap tank was positioned just above the slurry level near the center of the agitator to maximize the retention time of the slurry on the blades. The supply pump and product flow pump were adjusted to maintain a constant level within the trap tank. Double-head peristaltic pumps were used for both the supply stream and the product flow stream.

The supply stream was started to fill the trap tank to the overflow port, and the product line was recycled to the trap tank during filling using a double-head pump. The agitator was powered on and set to rpm, as outlined in the table below. Airflow was supplied in some experiments, as identified in the table below. No airflow was found to aid separation, as illustrated in the table below. This table also shows the operating time. In each experiment, the supply and product streams flowed for approximately three holding cycles. After the final pass, the product was collected in a clean beaker. The amount of blackchar present in the product was qualitatively determined by observing the product in the beaker and comparing it to that in the trap tank and overflow tank.

All operations utilize a 1.65-liter trap tank with a 4.9-inch inner diameter and either a 4.8-inch diameter flat-blade or 4.0-inch diameter cowl-blade agitator. These operations demonstrate the need for low rpm agitation and slow feed rates.

Example 6: Removal of blackchar by ozone treatment A lime mud slurry with a 10% solid content was prepared from the lime mud by dilution with water.

Approximately 625 ml of lime sludge slurry was poured into a 1-liter bottle with a hole in the lid for an R100 agitator and another hole at the top for supplying ozone via an SS tube. The slurry was mixed at 730 rpm. The valve of the dry air cylinder was opened and supplied to the ozone generator at a pressure of 10 psi. A dry air stream was supplied at 1 lpm. The ozone generator (Model 1 KNT purchased from Oxidation Technologies) was operated at 100% ozone level. While not intended to be constrained by theory, it is believed that ozone reacts with blackchar to oxidize it to carbon dioxide, which is then removed by the gas stream.

After processing, the dry brightness was tested using Hunter and ISO standards, photographs of the slurry surface were taken, and the % black spot area was determined using ImageJ software. TGA-DSC analysis was performed.

Example 7: Removal of black char using flotation A lime mud slurry with a solid content of 10% was prepared by diluting a filter cake with a solid content of approximately 70% with water.

A kerosene collector at a concentration of 200 or 500 ppm was added to the slurry and mixed for 2 minutes.

Next, 100 or 250 ppm of MIBC (methyl isobutylcarbinol) foaming agent was added, and the mixture was mixed for 0.5 minutes.

Next, the mixture was mixed at 2000 or 1500 rpm under an airflow of 4 standard liters per minute. Mixing and aeration were continued for 5 or 10 minutes, collecting any foam that overflowed.

The collected foam was tested for brightness as an indicator of black char content. The remaining tail slurry contained purified _CaCO3 with reduced black char.

The use of "a" or "an" is used to describe elements and components of the embodiments herein. This is done for convenience and to give a general meaning to the description.
This statement should be interpreted as including one or at least one, and unless it is clear that there is another meaning, the singular form also includes the plural form.

Furthermore, these figures illustrate preferred embodiments for illustrative purposes only. Those skilled in the art will readily recognize from the following considerations that alternative embodiments of the structures and methods illustrated herein may be used without departing from the principles described herein.

Therefore, while specific embodiments and applications are illustrated and described, it should be understood that the disclosed embodiments are not limited to the exact structures and components disclosed herein. Various modifications, changes, and variations, which will be apparent to those skilled in the art, can be made to the configuration, operation, and details of the methods and apparatus disclosed herein without departing from the spirit and scope defined in the appended claims.

Claims (12)

A method for producing purified precipitated calcium carbonate from lime sludge,
The lime mud cake is mixed with water and a dispersant to form a first slurry having a Brookfield viscosity of less than 1000 cps at 100 rpm,
The first slurry is pulverized to a median particle size of 0.4 microns to 5 microns ,
The process involves separating the phases of the pulverized slurry under conditions sufficient to obtain a centrifugal slurry containing impurity particles and a paste containing purified calcium carbonate,
A method comprising diluting the paste in water to a target solid content, thereby producing a dispersed slurry containing the purified precipitated calcium carbonate. The method according to claim 1, further comprising adjusting the pH of the dispersed slurry to a pH of 8 to 10 , and/or adjusting the pH of the pulverized slurry to a pH of 8 to 10 . The method according to claim 1 or 2, wherein the phase separation comprises centrifuging the pulverized slurry with a force of 500 g to 2000 g over a residence time of 1 to 10 minutes . The method according to claim 1 or 2, wherein the phase separation includes allowing the pulverized slurry to settle by gravity. The method according to claim 4, wherein the gravity sedimentation is carried out over a sedimentation period of 2 to 8 hours to a sedimentation depth of 2 cm to 3000 cm . The method according to claim 1, further comprising: mixing the lime clay cake with water to form a first slurry before forming the dispersed slurry; and washing the first slurry to remove the caustic soda present in the lime clay cake. The method according to any one of claims 1 to 6, wherein the first slurry has a solid content of 25% to 50% by weight . A method for producing purified precipitated calcium carbonate from lime sludge,
The lime mud cake is mixed with water to form a first slurry,
The pH of the first slurry is adjusted to be between 10 and 11 .
Under conditions sufficient to achieve a g-force of 500 to 2000 g , the first slurry is centrifuged over a residence time of 1 to 10 minutes to obtain a centrifugal slurry containing impurity particles and a paste containing purified calcium carbonate.
A method comprising: mixing the paste with water and a dispersant to form a dispersion slurry having a Brookfield viscosity of less than 1000 cps at 100 rpm and containing the purified precipitated calcium carbonate. The method according to claim 8, further comprising grinding the dispersed slurry to a median particle size of 0.4 microns to 5 microns . The method according to claim 9, further comprising adjusting the pH of the pulverized slurry to 9 to 10.5 . The method according to any one of claims 1 to 10, further comprising adjusting the pH of the dispersion slurry to 9 to 10.5 . The method according to any one of claims 8 to 11, wherein the first slurry has a solid content of 10% to 25% .